Percussive drilling is an established technique designed to break rock by impact forces transferred from the drill string to the drill bit at the bottom of a borehole. Typically, the energy required to break the rock is generated by a pneumatic or hydraulic actuated hammer mounted at a ground level end of the string. In particular, a pressured piston is driven forward to contact a shank adaptor at the end of the string such that kinetic energy of the piston is translated to a stress (or shock) wave that travels through the drill string to the drill bit mounted at the furthest end of the string. To maximise energy transfer efficiency, energy loss due to reflections between the coupled drill string members should be minimised.
In particular, it is well established that impedance mismatch in a percussive drill system will reduce the transmitted energy due specifically to reflections that occur at each threaded coupling. For example, a typical energy wave loses an appreciable percentage of its energy each time it passes through a coupling. This loss is due partly to the difference in cross sectional area between the threaded male and female connectors and partly to the imperfect contact between the rod ends. Efficiency of the percussive drilling of rock with regard to the drill rod joints is discussed by: B. Lundberg: “Efficiency of percussive drilling with extension rods”, International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, Vol. 24, no. 4, 1 Aug. 1987, pages 213-222, ISSN: 0148-9062, DOI: 10.1016/0148-9062(87)90176-8; and also by E. Beccu ET AL: “Efficiency of percussive drilling of rock with dissipative joints”, International Journal of Impact Engineering, vol, 9 no. 3, 1 Jan. 1990, pages 277-287, ISSN: 0734-743X, DOI: 10.1016/0734-743X(90)90003-E. Example drill string systems having threaded end connections are described in U.S. Pat. No. 6,485,061 and U.S. 2006/0032629.
Non percussive drill systems also utilise rods having threaded ends with example rods described in: Anonymous: “Drill Rod & Casing Dimensions”, 1 Dec. 2012, pages 1-1: http://www.mobiledrill.net/page/drill-rod-and-casing-dimensions; “Tool Joint Dimensional Data”, 1 Dec. 2003, pages 1-3: “ROTARY SHOULDER HANDBOOK”, 1 Nov. 2011, pages 1-116: http//www.nov.com/uploadedFiles/Business_Groups/Grant_Prideco/Drilling_Tubulars/Catalog/D392002466-MKT-001 Rev 02 Rotary Shoulder Handbook RS.pdf.
To optimise drilling performance, drilling parameters associated with percussion pressure, feed force and rotation are specifically selected by an operator. However, the maximum efficiency is limited by the impedance mismatch of the couplings. Moreover, the maximum shock wave amplitude is restricted by the configuration of the male connector. There is therefore a need to improve the energy transmission between the coupling regions of a drill string system.